Sherlock Holmes

on

Deduction

and

Deductive Reasoning

Gleaned from the stories of Sherlock Holmes written by Sir Arthur Conan Doyle are these quotes from Mr. Sherlock Holmes regarding his techniques and methods for solving mysteries and crimes using deductive reasoning.

Simply Stated – Deduction is Reasoning Backwards

“In solving a problem of this sort, the grand thing is to be able to reason backwards. That is a very useful accomplishment, and a very easy one, but people do not practise it much. In the every-day affairs of life it is more useful to reason forwards, and so the other comes to be neglected. There are fifty who can reason synthetically for one who can reason analytically…Let me see if I can make it clearer. Most people, if you describe a train of events to them, will tell you what the result would be. They can put those events together in their minds, and argue from them that something will come to pass. There are few people, however, who, if you told them a result, would be able to evolve from their own inner consciousness what the steps were which led up to that result. This power is what I mean when I talk of reasoning backwards, or analytically.”

Sherlock Holmes Quote

-A Study in Scarlet

-Chapter 7 – Conclusion

Do Not Theorize Before Gathering Data

“It is a capital mistake to theorize before you have all the evidence. It biases the judgment.”

Sherlock Holmes Quote

-A Study in Scarlet

“It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.”

Sherlock Holmes Quote

-A Scandal in Bohemia

“Still, it is an error to argue in front of your data. You can find yourself insensibly twisting them round to suit your theories.“

Sherlock Holmes Quote

-The Adventure of Wisteria Lodge

Suggested by: Bill Saunders

“Let me run over the principal steps. We approached the case, you remember, with an absolutely blank mind, which is always an advantage. We had formed no theories. We were simply there to observe and to draw inferences from our observations.”

“I had,” he said, “come to an entirely erroneous conclusion, my dear Watson, how dangerous it always is to reason from insufficient data.”

Sherlock Holmes Quote

-The Adventure of the Speckled Band

Data

“There is nothing like first-hand evidence.”

Sherlock Holmes Quote

-A Study in Scarlet

Notice Trifles

“You know my method. It is founded upon the observation of trifles.”

Sherlock Holmes Quote

-The Bascombe Valley Mystery

“They say that genius is an infinite capacity for taking pains,” he remarked with a smile. “It’s a very bad definition, but it does apply to detective work.”

Sherlock Holmes Quote

-A Study in Scarlet

“It has long been an axiom of mine that the little things are infinitely the most important.”

Sherlock Holmes Quote

-A Case of Identity

Obvious Facts Can be Deceptive

“There is nothing more deceptive than an obvious fact.”

Sherlock Holmes Quote

-The Bascombe Valley Mystery

Don’t Just See, Observe!

“The world is full of obvious things which nobody by any chance ever observes.”

Sherlock Holmes Quote

-The Hound of the Baskervilles

Chapter 3: “The Problem

‘You see, but you do not observe. The distinction is clear.’

Sherlock Holmes Quote

-A Scandal in Bohemia

Avoid Emotion

“Detection is, or ought to be, an exact science, and should be treated in the same cold and unemotional manner.”

Sherlock Holmes Quote

-The Sign of Four

‘The emotional qualities are atagonistic to clear reasoning.’

Sherlock Holmes Quote

-The Sign of Four

Recognize Vital Facts

“It is of the highest importance in the art of detection to be able to recognize, out of a number of facts, which are incidental and which vital. Otherwise your energy and attention must be dissipated instead of being concentrated.”

Sherlock Holmes Quote

-The Reigate Puzzle

Eliminate the Impossible and What Remains Is Truth

“Eliminate all other factors, and the one which remains must be the truth.”

Sherlock Holmes Quote

-The Sign of Four

Chapter 1: “The Science of Deduction”

“How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?”

Sherlock Holmes Quote

-The Sign of Four

‘…when you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth.’

Sherlock Holmes Quote

-The Blanched Soldier

—

‘It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth.’

Sherlock Holmes Quote

-The Beryl Coronet

Additional Notes

“Nothing clears up a case so much as stating it to another person.”

Sherlock Holmes Quote

-Silver Blaze

—

“I have already explained to you that what is out of the common is usually a guide rather” than a hindrance.”

Sherlock Holmes Quote

-A Study in Scarlet

-Chapter 7 – Conclusion

—

‘”The more outre’ and grotesque an incident is the more carefully it deserves to be examined, and the very point which appears to complicate a case is, when duly considered and scientifically handled, the one which is most likely to elucidate it.”

Sherlock Holmes

-The Hound of the Baskervilles

Chapter 15 – “A Retrospection”

—

“Any truth is better than indefinite doubt.”

Sherlock Holmes Quote

-The Yellow Face

—

“I never guess. It is a shocking habit — destructive to the logical faculty”

Sherlock Holmes Quote

-The Sign of Four

Sherlock Holmes Expounds on Logic, Inference and Deduction

Then I [Dr. Watson] picked up a magazine from the table and attempted to while away the time with it, while my companion munched silently at his toast. One of the articles had a pencil mark at the heading, and I naturally began to run my eye through it.

Its somewhat ambitious title was “The Book of Life,” and it attempted to show how much an observant man might learn by an accurate and systematic examination of all that came in his way. It struck me as being a remarkable mixture of shrewdness and of absurdity. The reasoning was close and intense, but the deductions appeared to me to be far-fetched and exaggerated. The writer claimed by a momentary expression, a twitch of a muscle or a glance of an eye, to fathom a man’s inmost thoughts. Deceit, according to him, was an impossibility in the case of one trained to observation and analysis. His conclusions were as infallible as so many propositions of Euclid. So startling would his results appear to the uninitiated that until they learned the processes by which he had arrived at them they might well consider him as a necromancer.

“From a drop of water,” said the writer, “a logician could infer the possibility of an Atlantic or a Niagara without having seen or heard of one or the other. So all life is a great chain, the nature of which is known whenever we are shown a single link of it. Like all other arts, the Science of Deduction and Analysis is one which can only be acquired by long and patient study nor is life long enough to allow any mortal to attain the highest possible perfection in it. Before turning to those moral and mental aspects of the matter which present the greatest difficulties, let the enquirer begin by mastering more elementary problems. Let him, on meeting a fellow-mortal, learn at a glance to distinguish the history of the man, and the trade or profession to which he belongs. Puerile as such an exercise may seem, it sharpens the faculties of observation, and teaches one where to look and what to look for. By a man’s finger nails, by his coat-sleeve, by his boot, by his trouser knees, by the callosities of his forefinger and thumb, by his expression, by his shirt cuffs—by each of these things a man’s calling is plainly revealed. That all united should fail to enlighten the competent enquirer in any case is almost inconceivable.”

“What ineffable twaddle!” I cried, slapping the magazine down on the table, “I never read such rubbish in my life.”

“What is it?” asked Sherlock Holmes.

“Why, this article,” I said, pointing at it with my egg spoon as I sat down to my breakfast. “I see that you have read it since you have marked it. I don’t deny that it is smartly written. It irritates me though. It is evidently the theory of some arm-chair lounger who evolves all these neat little paradoxes in the seclusion of his own study. It is not practical. I should like to see him clapped down in a third class carriage on the Underground, and asked to give the trades of all his fellow-travellers. I would lay a thousand to one against him.”

“You would lose your money,” Sherlock Holmes remarked calmly. “As for the article I wrote it myself.”

“You!”

“Yes, I have a turn both for observation and for deduction. The theories which I have expressed there, and which appear to you to be so chimerical are really extremely practical—so practical that I depend upon them for my bread and cheese.”

“And how?” I asked involuntarily.

“Well, I have a trade of my own. I suppose I am the only one in the world. I’m a consulting detective, if you can understand what that is. Here in London we have lots of Government detectives and lots of private ones. When these fellows are at fault they come to me, and I manage to put them on the right scent. They lay all the evidence before me, and I am generally able, by the help of my knowledge of the history of crime, to set them straight. There is a strong family resemblance about misdeeds, and if you have all the details of a thousand at your finger ends, it is odd if you can’t unravel the thousand and first.

Sherlock Holmes Quote

-The Sign of Four

Chapter 2 – “The Science of Deduction”

All quotes on this web site were written by Sir Arthur Conan Doyle unless otherwise stated.